Just popping by to say, I used this, though in a slightly different fashion. Rather than using additive rotation on the foot poses, I use a world rotation specified by the target rotation of the bone from the anim, combined with the normal rot from the trace. The advantage to this is that poses work better extended to extreme lengths (otherwise, when using additive, a very extended leg tends to rotate the foot itself since that’s a child bone; by forcing the rotation to always be the target rotation of the anim, offset by the surface rotation, in situations where the leg is bent in the extreme, the foot’s position relative to the anim on flat ground is consistent. If it were pure additive, we’d add the rotation to the anim’s rotation, which is already offset BY the leg rotation, sometimes leading to disconnected toes or weird rest positions for feet).
I also forewent the BP-driven on-off for each foot’s IK, and instead created three variable curves which all my anims use; LeftFootIK, RightFootIK, and TorsoIK; these all contain a 0-1 range which adjusts how much the IK should weight in for each frame of an anim (the TorsoIK doesn’t alpha blend the location adjust, just a rotation adjust created from the average of the two foot traces; this way, for things like running down steep hills or rolls/other movements which are surface-dependent, I can blend the entire skeleton into a rotation with the surface, then blend out to a proper upright where only the feet/legs are offset). Using weights works nicely for the feet; for things like melee attacks, where a foot might lift slightly off the surface, I can go to a weight of 0.5 or 0.8 or thereabouts, which lets the foot “lift” with the anim but still causes that lift to track the surface somewhat, avoiding situations where lifted legs might clip through steep terrain.
It works remarkably well, as long as you’re willing to fine-tune each anim on slopes of various steepness from all directions to make sure the blend weights always feel right.